Daily transient decreases in plasma parathyroid hormone levels induced by the calcimimetic NPS R-568 slows the rate of bone loss but does not increase bone mass in ovariectomized rats

New therapies: calcimimetics, phosphate binders and vitamin D receptor activators

At present, new compounds are available to treat secondary hyperparathyroidism, namely calcimimetics, novel phosphorus binders and also novel vitamin D receptor activators. Calcimimetics increase the sensitivity of the parathyroid gland to calcium through spatial configurational changes of the calcium-sensing receptor. In addition, experimental studies have demonstrated that calcimimetics also upregulate both the calcium-sensing receptor and the vitamin D receptor. They are efficacious in children, though the experience in paediatric chronic kidney disease is still limited. Sevelamer, lanthanum carbonate and magnesium iron hydroxycarbonate are novel phosphorus binders available on the market. Several studies have demonstrated their efficacy and safety up to 6 years, though costs are the main limitation for a wider use. Since almost all the experience available on the new phosphorus binders comes from its use in adults, studies on children are needed in order to confirm the efficacy and safety of these products. Other new salts and polymers are also being developed. New vitamin D receptor activators, such as paricalcitol, are as effective at suppressing parathyroid hormone (PTH) as the traditional vitamin D receptor activators used for the past two decades, but they have a better and safer profile, showing fewer calcaemic and phosphoraemic effects while preserving the desirable effects of the vitamin D receptor activators on the cardiovascular system, hypertension, inflammation and fibrosis. Their use in children with chronic kidney disease has revealed similar responses to those of adults. The novel compounds discussed in this review should facilitate and improve the management of mineral and bone disorders in children with chronic kidney disease.

The calcium-sensing receptor in bone cells: a potential therapeutic target in osteoporosis

Recent progress has been made in our understanding of the functional role of the seven-transmembrane-spanning extracellular calcium-sensing receptor (CaSR) in bone cells. Both in vitro and in vivo data indicate that the CaSR is a physiological regulator of bone cell metabolism. The CaSR regulates the recruitment, differentiation and survival of osteoblasts and osteoclasts through activation of multiple CaSR-mediated intracellular signaling pathways in bone cells. This raises the possibility that the bone CaSR could potentially be targeted by allosteric modulators, either agonists (calcimimetics) or antagonists (calcilytics) to control bone remodeling. The therapeutic potential of CaSR agonists or antagonists in bone cells is however hampered by their effects on the CaSR in nonskeletal tissues. Rather, direct targeting of the bone CaSR may be of potential interest for the treatment of bone diseases. Targeting the bone CaSR using a bone-seeking CaSR agonist offers a potential mean to modulate bone cell metabolism. The development of drugs that preferentially target the CaSR and possibly other cation-sensing receptors in bone cells may thus be helpful for the treatment of osteoporosis.

An orally active calcium-sensing receptor antagonist that transiently increases plasma concentrations of PTH and stimulates bone formation

Daily subcutaneous administration of exogenous parathyroid hormone (PTH) promotes bone formation in patients with osteoporosis. Here we describe two novel, short-acting calcium-sensing receptor antagonists (SB-423562 and its orally bioavailable precursor, SB-423557) that elicit transient PTH release from the parathyroid gland in several preclinical species and in humans. In an ovariectomized rat model of bone loss, daily oral administration of SB-423557 promoted bone formation and improved parameters of bone strength at lumbar spine, proximal tibia and midshaft femur. Chronic administration of SB-423557 did not increase parathyroid cell proliferation in rats. In healthy human volunteers, single doses of intravenous SB-423562 and oral SB-423557 elicited transient elevations of endogenous PTH concentrations in a profile similar to that observed with subcutaneously administered PTH. Both agents were well tolerated in humans. Transient increases in serum calcium, an expected effect of increased parathyroid hormone concentrations, were observed post-dose at the higher doses of SB-423557 studied. These data constitute an early proof of principle in humans and provide the basis for further development of this class of compound as a novel, orally administered bone-forming treatment for osteoporosis.

Therapeutic use of calcimimetics

It has long been recognized that the secretion of PTH by chief cells in the parathyroid gland is regulated by extracellular ionized calcium. The molecular mechanism by which extracellular Ca2+ performs this feat was deduced by the cloning of the extracellular calcium-sensing receptor (CaSR) in 1993 in the laboratories of Brown and Hebert. The CaSR is a G protein-coupled cell surface receptor that belongs to family 3 of the GPCR superfamily. The CaSR senses the extracellular ionic activity of the divalent minerals Ca2+ and Mg2+ and translates this information, via a complex array of cellular signaling pathways, to modify cell and tissue function. Genetic studies have demonstrated that the activity of this receptor determines the steady-state plasma calcium concentration in humans by regulating key elements in the calcium homeostatic system. CaSR agonists (calcimimetics) and antagonists (calcilytics) have been identified and have provided both current and potential therapies for a variety of disorders. Calcimimetics can effectively reduce PTH secretion in all forms of hyperparathyroidism. They are likely to become a major therapy for secondary hyperparathyroidism associated with renal failure and for treatment of certain patients with primary hyperparathyroidism. On the therapeutic horizon are calcilytics that can transiently increase PTH and may prove useful in the treatment of osteoporosis.

Structural organization and biological relevance of oscillatory parathyroid hormone secretion

Parathyroid gland secretory activity exhibits seasonal and circadian fluctuations, which are in synchrony with changes in serum calcium, phosphate, and bone turnover. In addition, an ultradian rhythm exists, which comprises seven pulses per hour, accounts for 30% of basal parathyroid hormone (PTH) release, and is highly sensitive to changes in ionized calcium. Acute hypocalcemia induces a selective, severalfold increase in pulse frequency and amplitude, whereas hypercalcemia suppresses the pulsatile secretion component, as does prolonged calcitriol therapy. Chronic renal failure is associated with a GFR dependent decrease in metabolic PTH clearance accounting for a two- to threefold increase in plasma PTH concentrations, a consistent increase of PTH burst mass and frequency, and a markedly reduced capacity to counteract changes in ionized calcium by modulation of pulsatile PTH release. Continuous PTH excess destroys bone, whereas intermittent administration of pharmacological doses of PTH improves bone morphology and strength in experimental and clinical settings. The molecular mechanisms of the exposure pattern dependent, contrasting biological effects of PTH may involve differential regulation of osteoblastic G protein signaling feedback circuits. In this context, calcimimetic and calcilytic agents are promising new therapeutic tools allowing for tight control of plasma PTH and restoration of circadian PTH rhythmicity.

Calcimimetics and the Treatment of Primary and Secondary Hyperparathyroidism

OBJECTIVE

To review the mechanism of action, development, and clinical application of the calcimimetic compounds being investigated for the treatment of primary and secondary hyperparathyroidism (HPT).

DATA SOURCES

A MEDLINE search (1990–April 2004) was performed to identify all published articles related to calcimimetics. Published abstracts over the previous 5 years from various scientific meetings (American Society of Nephrology, American Society for Clinical Pharmacology and Therapeutics, American Society of Bone and Mineral Research) were also searched for reports regarding investigational calcimimetic agents. Data on cinacalcet HCI were provided by Amgen, Inc.

STUDY SELECTION AND DATA EXTRACTION

Studies were selected based on number of patients included, relevance to the approved indications, and inclusion of pharmacokinetic and drug interaction information.

DATA SYNTHESIS

The investigational calcimimetic compounds directly modulate the calcium-sensing receptor and can produce at least a 30% reduction in parathyroid hormone (PTH) secretion in secondary HPT. Cinacalcet appears to have more predictable pharmacokinetics and a lower risk of symptomatic hypocalcemia than the older agent, R-568. The safety profile and effective reduction in PTH and simultaneous reductions in calcium and phosphorus (and the calcium–phosphorus product) induced by cinacalcet make this agent advantageous over previously used therapies, such as vitamin D and phosphate-binding agents. More studies are required to determine additional uses of cinacalcet.

CONCLUSIONS

Cinacalcet has a unique mechanism for reducing PTH concentration and appears to be a safe and effective oral therapy for both primary and secondary HPT.

The physiology of the circadian rhythm of parathyroid hormone and its potential as a treatment for osteoporosis

PURPOSE OF REVIEW

A circadian rhythm exists for parathyroid hormone, with a biphasic pattern showing a late afternoon/early evening rise and fall and a broader, longer-lasting increase late evening/early morning reaching nadir mid-morning. This review explores the characteristics of the circadian rhythm, factors regulating the rhythm and its role in bone metabolism.

RECENT FINDINGS

Gender differences exist in the circadian rhythm for parathyroid hormone. Ageing in women alters the response to calcium infusion, increasing the suppression of parathyroid hormone secretion and decreasing bone resorption. There is no difference between young and elderly men in the parathyroid hormone response to calcium infusion. Loop diuretic ingestion alters the parathyroid hormone circadian rhythm reflecting loop diuretic effects on phosphate and calcium metabolism. Adult growth hormone deficiency alters parathyroid hormone secretion and end organ sensitivity, but the circadian rhythm is retained. Growth hormone replacement therapy enhances the parathyroid hormone circadian rhythm and increases end organ responses. Exogenous parathyroid hormone (1-34) and (1-84) administered by daily injection has an anabolic effect on bone, increasing bone mass and decreasing fracture. Calcilytic drugs stimulate and calcimimetic drugs suppress parathyroid hormone secretion and have been used to treat disorders of bone metabolism.

SUMMARY

The circadian nature of parathyroid hormone secretion is confirmed by many publications. The underlying rhythm is endogenous. Life style factors and nutritional intake modulate the pattern of secretion. Direct association with bone resorption and formation is tentative. It is suggested that acute changes in these rhythms have little effect on resorption, but longer-term manipulation of parathyroid hormone secretion alters bone cell function. Growth hormone therapy in adult deficiency increases parathyroid hormone activity, indicating growth hormone may have therapeutic potential for osteoporosis. Manipulation of the endogenous parathyroid hormone rhythm, using timed supplements of phosphate or calcium or by calcilytic and calcimimetic molecules, offers a novel approach to osteoporosis treatment.

Skeletal abnormalities in Pth-null mice are influenced by dietary calcium

We have examined the role of PTH in the postnatal state in a mouse model of PTH deficiency generated by targeting the Pth gene in embryonic stem cells. Mice homozygous for the ablated allele, when maintained on a normal calcium intake, developed hypocalcemia, hyperphosphatemia, and low circulating 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] levels consistent with primary hypoparathyroidism. Bone turnover was reduced, leading to increased trabecular and cortical bone volume in PTH-deficient mice. When mutant mice were placed on a low-calcium diet, renal 25-hydroxyvitamin D 1 alpha-hydroxylase expression increased despite the absence of PTH, leading to a rise in circulating 1,25(OH)(2)D(3) levels, marked osteoclastogenesis, and profound bone resorption. These studies demonstrate the dependence of the skeletal phenotype in animals with genetically depleted PTH on the external environment as well as on internal hormonal and ionic circulatory factors. They also show that, although PTH action is the first defense against hypocalcemia, 1,25(OH)(2)D(3) can be mobilized, even in the absence of PTH, to guard against extreme calcium deficiency.